Next generation optical networks are focused on economically exploiting the inherent bandwidth of optical fiber. Optical routing, agile wavelength provisioning and wavelength management are key characteristics of these next generation communications systems. The ability to perform attenuation and switching on a per wavelength basis is an enabling technology which achieves enhanced network flexibility.
Optical communication systems impose a number of particular and demanding requirements on DWDM wavelength filters and network devices. The optical requirements, such as channel extinction and isolation, chromatic dispersion, polarization dependent loss, passband width, passband flatness and insertion loss are becoming increasingly demanding as data rates increase and channel spacings are reduced. For example, at 25 and 50 GHz separations, prior art filtering and add/drop components have difficulty in meeting the optical performance requirements of next generation all-optical networks. For these applications, cost, compactness and performance characteristics such as dynamic range and extinction ratio are of great importance. In addition the response time must also meet the performance criteria over a given range of temperatures.
Unique wavelength management functionality can be achieved by combining diffraction grating and liquid crystal array technology in a dynamic channel equalizer/blocker system. This approach allows completely independent attenuation and/or blocking control on a per channel basis for 25, 50 and/or 100 GHz channel spacings. The introduction of a dynamic multi-cell LC-SLM array to modulate the wavelength demultiplexed beams enables devices which perform switching and variable attenuation in a channel independent fashion.
At present, the performance of channel equalizers is limited by the fact that liquid crystal spatial light modulators commonly used for fiberoptic applications exhibit wavelength and temperature dependence. For visible display applications, compensator films have been developed to achromatize the liquid crystal response across the visible light spectrum. The performance requirements for this application include achromatic contrast (>20 dB) for a relatively wide spectral band of 400 to 700 nm. However, the numerous designs developed for display applications are not particularly relevant to the needs of telecom applications. For instance, dynamic switching and variable attenuation products demand >40 dB extinction in a wavelength independent fashion across the C, L, or S bands. These bands are approximately 35 nm wide, within the 1300 to 1700 nm wavelength range. The extension of visible display techniques to achromatize infrared LC-SLMs does not provide the performance required for these unique fiberoptic applications.
In the prior art, the relationship between the control voltage on each LC-SLM pixel and attenuation set point depends on the temperature T and wavelength λ; that is, V(pixel N)=V(pixel N, attenuation,T, λ). Functionally, the control processing required for each pixel requires several control inputs. This requires a lookup table to be implemented into the driver electronics to correct for these first order errors. The generation of the entries for this look-up table requires extensive optical testing under different drive voltage, temperature, and wavelength conditions. Since this four-dimensional parameter space cannot practically be sampled over a finite number of discrete intervals, the response must be interpolated to fill in the missing data. This interpolation further reduces the accuracy of the system performance. Therefore, the goal of this invention is to make V(pixel N)=V(attenuation) to first order. As a result, two options arise: the lookup table is not needed, or the lookup table is used to correct second order rather than first order errors. This second option provides much improved accuracy in controlling the attenuation of each pixel.
Numerous factors relating to the intrinsic properties of liquid crystals and compensators, their optical axes and orientations, and the retardations they introduce, are known to affect the response of a device in a network. A channel equalizer/blocker provides control of the attenuation/extinction on a per channel basis. Optical networks require that channel blocking be achieved to a >40 dB rejection level. This level of blocking places extreme demands on the precision in which the retardation of individual LC-SLM pixels must be controlled. Typically, a compensator plate is placed in front of the LC cell to properly bias the retardation by half wave (π radians or 725 nm) in round trip. However, this compensator plate, in addition to the LC-SLM, introduces a well characterized sinusoidal wavelength dependence with free spectral range given by c/Γ, where Γ is the retardation and c is the speed of light. For a half wave plate, Γ=π, and for a quarter wave plate, Γ=π/2.
An optimal liquid crystal cell design to realize variable attenuation and switching for dynamic channel equalizers is based on the reflective, counter propagating configuration described in an earlier U.S. patent application Ser. No 10/209,879, filed Aug. 2, 2002 by A. S. Kewitsch et al and entitled “Liquid Crystal Modulator and Polarization Diversity Optics for Optical Communications”. This is the zero twist nematic, parallel aligned liquid crystal in the reflective electrically controlled birefringence (ECB) mode. This type of cell is driven by a square wave voltage signal of 4 to 20 KHz frequency at 0 to 10 volt amplitude. This elemental cell design alone provides maximum retardation at low voltage (i.e. 0 volts), and minimum retardation (i.e. <10 nm) at high voltage (>3 volts). The cells are typically 50 to 100 um wide along the array direction, as defined by the dispersion direction of the diffraction grating, and are 1 to 3 mm tall.
The total retardation of a reflective LC-SLM is the vector sum of the liquid crystal and the compensator plate retardation, multiplied by two because of the round trip reflection. For use of the cell in the parallel polarizer type amplitude modulator, the liquid crystal should provide between 0 and π radians of phase retardation (0 to half wave) in round trip. Therefore, the liquid crystal cell provides 0 to quarter wave retardation in a single pass. From a fault handling perspective, telecom applications may require, for fail-safe operation, that a variable attenuator/blocker provide nominally full transmission in the zero voltage or power off state. This requires in turn that the retardation is zero rather than quarter wave in the zero voltage state. To achieve this normally-on type response, a nominally quarter wave compensator in antiparallel alignment to the optical axis of the LC cell is typically placed in front of the LC-SLM to shift the total LC retardation downward by slightly more than half wave, which biases the net zero voltage retardation about zero. The optical axes of the quarter wave plate and the LC director are in precise parallel alignment to one another. Therefore, in the high extinction state, the retardation is provided almost exclusively by the compensator plate. This compensator plate is typically fabricated from quartz of 45 to 50 um thickness. In the low voltage state, the sum of the retardation of the LC-SLM and the compensator plate equals an integer multiple of a full wave, the multiple being typically zero or one. As described earlier, when the cell is aligned such that the fast axis of the compensator is parallel to the slow axis of the liquid crystal cell, then this multiple is zero.
The retardation in the high extinction state is provided primarily by the true zero order quartz compensator plate of birefringence (retardation) equal to quarter wave plus an additional amount of birefringence (e.g., 0.1 wave) to correct for the residual birefringence of the cell in the high voltage state. The thickness of this waveplate is typically 54 um for quartz. However, a zero order wave plate introduces significant wavelength dependence of the voltage required for extinction across the C-band or L-band. The voltage required to achieve high extinction can vary by 20% in a linear fashion from 1530 to 1565 nm.
Since the retardation is provided by the sum of the liquid crystal retardation and the compensator retardation, the temperature dependence of the cell is equal to the sum of the temperature dependence of the liquid crystal and the compensator. Liquid crystal materials typically exhibit larger thermooptic and thermal expansion coefficients than quartz. Therefore, it is advantageous if the retardation in the high extinction state is provided primarily by the compensator plate for all wavelengths. This can be achieved by utilizing the wavelength flattened compensator plate design described herein. In contrast, the compensator plate/SLM combination of the prior art produces an attenuation response which depends on wavelength, voltage, and temperature. The amount of retardation necessary to achieve the desired polarization transformation depends on the operating wavelength, so different wavelength channels require different voltage settings to achieve the same attenuation value. The wavelength dependence of extinction within the C band varies, for example, between 1565, 1545, and 1530 nm, which adds complexity in the look up table algorithms and drive electronics, compromising the precision in which the attenuation and extinction can be set across a wide range of temperatures and wavelengths. It also requires drive electronics which output a larger voltage range for the longer wavelength channels. This reduces the flexibility in the design of the overall system hardware, because drive electronics components are available for output voltages of 5, 8, 10, and 22 volts, wherein increasing the output voltage increases cost, size, power consumption and reduces the range of component selection.